indicate that the sources are not yet entirely characterized. For example, N20 in the lower stratosphere has been shown to be isotopically enriched in both 18 O and 15 N relative to tropospheric N20 (Kim and Craig, 1993). N20 emissions from tropical rain forest soils, fertilized soils, and a wastewater treatment facility are lighter in both of these heavy isotopes than tropospheric NzO (Yoshinari and Wahlen, 1985; Wahlen and Yoshinari, 1985; Kim and Craig, 1990, 1993). Either there is a source of N20 in the stratosphere that selectively produces heavy N20 or one that in the stratosphere selectively destroys light N20 (Johnson et al., 1995). Despite a number of studies, the source of this discrepancy is not yet clear (McElroy and Jones, 1996; Wingen and Finlayson-Pitts, 1998; Cliff and Thiemens, 1997; Rahn and Wahlen, 1997), although such processes as enhanced photolysis of the l4Nl4Nl60 isotopomer (Yung and Miller, 1997; Rahn et al., 1998) and/or formation of N20 by reaction of highly vibrationally excited 03 with N2 (Zipf and Prasad, f998) have been proposed.

One interesting potential source of N20 is the heterogeneous oxidation of HONO on surfaces (Wiesen et al., 1995; Pires and Rossi, 1997), which has been observed to form NzO. This is likely responsible for the observation of significant amounts of N20 in automobile exhaust, which was shown to be an artifact of sampling (Munzio and Kramlich, 1988). However, it may also occur on aerosol particles in the atmosphere (Clemens et al., 1997), an area that warrants further investigation.

Like C02 and CH4, the concentrations of N20 in the atmosphere have also been increasing, from ~275 ppb in the preindustrial era to ~312 ppb in 1994 (IPCC, 1996). Figure 14.19, for example, shows one set of measurements of N20 over the past 250 years obtained using ice core samples from Antarctica